Set artful as default Ubuntu for robot/qemu
Set artful as default Ubuntu for robot/qemu

While setting up a new system for CI, it was found that ubuntu:latest
now points to 18.04. There are issues with pip and ubuntu 18.04. There
are some workarounds but lets wait for the bugs to settle before doing
too much work here.

Here are some references on this issue:
https://github.com/pypa/pipenv/issues/2122
https://bugs.launchpad.net/ubuntu/+source/python-pip/+bug/1772746

Testing: Verified this now builds correctly on old and new systems

Change-Id: Ifed8b460f3df14aec7ba8f566291af87803c1e26
Signed-off-by: Andrew Geissler <f7919ac9f8ec0c3ffbc3f483da625c634fd31638@yahoo.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts